As is well-known, gas turbine engines that utilize axial flow compressors are subject to stall and surge. Stall may occur in the compressor when the angle of attack and other conditions are such that the boundary layer of the air adjacent to the compressor blades separates inducing a pressure pulsation. If the pulsation does not subside and is allowed to propagate to other blades, the entire compressor will surge which could lead to an engine malfunction. The industry has attempted to eliminate surge or provide means for insuring that surge will not ensue, and if so, in situ, a remedy is designed to obviate the condition.
Historically, fuel controls are designed to provide an open loop schedule that has sufficient surge margin to assure that the engine can be accelerated without incurring surge. The accepted philosophy for such schedules is to provide sufficient margin between the engine operating line and the surge line at the worst operating condition so that no matter what engine condition is encountered, surge will be avoided. The margin provided using this philosophy is a compromise between the rate of acceleration which could be achieved under the safest operating conditions and the surge margin required for the worst operating conditions. Since acceleration time is always sacrificed in favor of avoiding surge, accelerations are not as rapid as possible when operating at conditions other than the worst possible combination. Of course, it is ideal to accelerate the engine as rapidly as possible, so that in this scenario any means that will assure avoidance of surge while allowing rapid acceleration at all operating conditions is a desirable objective in this art.
Since the surge margin required for acceleration is normally dictated by the most severe operation the engine may encounter (even though that situation hardly arises, if ever), it is quite apparent that the engine operation can be enhanced at most operating conditions merely by ignoring the worse case scenario. Obviously, such is an unacceptable solution to the problem, since surge must be avoided at all operating conditions to assure flight safety.
As is well-known, fuel controls such as the JFC-12, JFC-60 and JFC-68 manufactured by the Hamilton Standard Division of United Technologies Corporation, the assignee of this patent application, provide open loop schedules with sufficient stall margins for avoidance of stall in all contemplated operations of the engines. For details of acceleration controls reference should be made to the aforementioned control models. Such control systems manifest a control parameter that is indicative of (where W.function./P.sub.B is fuel flow rate in pounds per hour and P.sub.B is burner pressure in pounds per square foot absolute). This parameter varies as a function of compressor speed (either the low compressor N.sub.1 or the high compressor N.sub.2) in a twin spool engine and other engine parameters selected to correct the speed to a base line value and is multiplied by actual burner pressure (P.sub.B) or its equivalent to schedule the proper fuel flow to the engine for acceleration.
Other engine control schemes may utilize a N.sub.1 or N.sub.2 (rate of change signal) to provide the same function as the W.function./P.sub.B parameter. But, in either instance or by a combination of the two, the stall margin is excessive and/or inherently provides slow accelerations when not operating under worst case conditions. Such inadequacies of these systems are acerbated even further when engine operations deviate from the norm due to power extraction, compressor bleed and engine efficiency degradation.
I have found that I can provide a control scheme that assures the optimum acceleration (most rapid) without risk of compressor stall with any combination of bleed, power extraction and engine condition. This invention contemplates a closed loop system which provides an acceleration control which generates a simulated compressor stall limit signal which is converted to a desired burner pressure limit. This limit is calculated by selecting a desired engine pressure ratio as a function of corrected high pressure compressor rotor speed and closing the loop on actual burner pressure to control fuel flow to the burner. The error between the actual burner pressure signal and simulated compressor stall limit signal determines the rate of fuel flow during acceleration, properly accounting for compressor bleed, power extraction and degradation of engine efficiency.